Mousedl1 I am sorry your dh had a shit relationship with his parents. And that he lost his mum and perhaps now feels he has no chance to make things right.
I fear that it is potentially this that has made him think it is reasonable to call you a selfish bitch and expect to dictate what you do, as a mother, on Mothering Sunday!
Whether Mothers' Day is important for you is totally up to you. It is important to me. As is Valentine's Day. And it doesn't matter why it is important to you.
I think your dh has to work out some things to do with the loss of his mum and the only thing I could recommend is some bereavement counselling.
I do fear potentially you will also need some relationship counselling, unless this is really an isolated incident.
As far as the day trip goes it does sound (sorry) very grim. Taking young children to an old people's home, especially of an old person they do not know, could be quite tricky.
I really feel he is wrong to leave you out of a family event like this but maybe this is the best way for him to cope with it.
When my husband's grandmother died he chose to go to the funeral without me, or our kids, for a variety of reasons. I did feel a bit left out but recognised it was absolutely the best thing. Had our kids been there he would have had to deal with them and their emotions and as this was the first death he had had to deal with as an adult (as far as I am aware) I did feel it was right.
So in your shoes I would cut him some slack that he is choosing to do this without you.
I would feel very upset by his comment about me.
I would absolutely not want him to plan what I could do on one of my rare days off.
You may feel angry with your SIL but to be honest, as someone who has lost two parents in the last 12 years, it does have a very bonding effect between siblings IMHO. Much as husbands and wives try and understand, they are just not as connected to the lost person. Even if the loss is one built around a sad/negative/unpleasant experience, it is something that maybe your husband and his sister share.
Good luck.